This invention relates generally to plasma probes, and more particularly to the construction and assembly of Langmuir probes used to measure properties of plasma, such as electron density and temperature.
FIG. 1 illustrates a conventional Langmuir probe. A typical Langmuir probe 1 includes an elongated shell 2 with a closed end 3 and an open end 4 and is constructed of a dielectric, such as a ceramic material. An electrically conductive contact 5 extends through the closed end of the shell. The contact 5 forms a node 7 (shown here as a male pin connector) positioned within the shell 1. An exposed portion of the contact 5 may be placed in direct contact with a plasma. A hermetic seal 6 may be positioned in the interior of the shell towards the sealed end to prevent plasma from entering the interior of the shell. The contact 5 extends through the hermetic seal 6, such that the tail end of the contact forms the male pin connector. Some probes have electrically conductive slugs 8 positioned around the contact and in the shell between the hermetic seal and the closed end of the shell to create a dielectric effect. In such probes, the contact 5 may be divided into forward and rearward sections 5a,5b with the slug 8 providing electrical conductance between the sections 5a,5b. 
A female pin connector 9 is detachably coupled to the node 7. Wiring W runs from the female pin connector 9 and through the open end 4 of the Langmuir probe 1. The free end of the wiring W can be attached to a measuring device which measures the potential created in the Langmuir probe 1. The wiring W is usually a coaxial cable with a nonconductive outer sheathing covering a braided wire shielding. The wiring W may be biased with potential from a power source. An RF inductor filter R may be coupled inline with the wiring W. The outer diameter of the filter R is smaller than the inner diameter of the shell 2 but may have an external diameter close to the internal diameter of the shell 2.
A conductive ring 11 may be provided around the shell 2 near the closed end 3 of the shell 2 to serve as a reference electrode. An electrically shielded grounding wire 13 is connected to the conductive ring 11. Also, an electrically conductive sleeve 17 may extend around the closed end 3 of the shell 2. A Conflat(copyright) fitting 15 extends around the Langmuir probe 1 towards the open end 4. The Conflat(copyright) fitting 15 seals against the container holding the plasma.
Semiconductor fabrication equipment often use plasma processing. Exemplary processes in which plasma is used are dry-etching of semiconductors for microcircuits and plasma enhanced chemical vapor deposition (CVD). When performing semiconductor etching and deposition, it is best to have uniformity of the ion current density in the plasma reactor chamber. Such uniformity can be created by measuring the density distribution of the plasma during testing and making adjustments to the plasma reactor chamber and the operating conditions. During fabrication, the ion current density can be checked, and if required, adjustments to reach uniformity may be made.
Langmuir probes 1 can be used to measure properties of plasma, such as when conducting testing and diagnostics in the processes described above. The electron density and temperature of plasma can be derived from Langmuir probe 1 measurements through the analysis of the current-potential characteristics of the plasma. The contact 5 of the Langmuir probe 1 is a conductor, and when placed in direct contact with moving charged particles found in the plasma, a current flows through the Langmuir probe 1. Based on the change in potential within the probe 1, an estimation of the temperature and density of the electrons in the plasma can be made.
During the measurement of the properties of the plasma, the Langmuir probe 1 heats up due to the current flowing through the wiring W and due to the exposure of the probe 1 to the plasma. Probe 1 heating can lead to deterioration of the probe 1 both mechanically and with respect to the RF filter R. Deterioration of the filter R can lead to total probe failure, or to a detuning of the filter R leading to high RF noise and resulting in inaccurate or misleading results. If the filter R becomes damaged and inoperable from the heat, the only remedy is to replace the filter R, which is a difficult and time consuming task.
Furthermore, the heat to which the probe 1 is exposed may cause the nonconductive shielding of the wiring W to melt, allowing core wires to come in contact with braided wire shielding and to cause a short. When this occurs, the wiring W must be replaced.
Yet another problem encountered with prior art Langmuir probes 1 is that the node 7 and female pin connector 9 becomes corroded due to the heat. Corrosion causes increased electrical resistance and must be removed for optimum electrical connectivity. However, because the contact 5 is fixed by the hermetic seal 6, the node 7 is accessible only through the interior of the shell 2. This makes removal of the oxidization very difficult, as cleaning must be accomplished through the open end 4 of the shell 2. Thus, the wiring W must be removed before cleaning can take place.
To replace the wiring W, the old wiring W, filter R, and female pin connector 9 are pulled from the shell 2 of the probe 1. New wiring, filter and female pin connector are then assembled. Generally, the wiring W is rigid enough to permit pushing of the female pin connector 9 into engagement with the node 7 during reassembly. However, because of the flexibility of the wiring W and the dimensions of the shell 2, i.e., very long with a small lumen, alignment of the female pin connector 9 with the node 7 so that they may be reattached is very difficult. The size of the filter R generally prohibits insertion of tools to guide the female pin connector 9. Thus, a great deal of time may be spent attempting to reattach the node 7 and female pin connector 9. Further, once the probe 1 is reassembled, there is a strong probability that the interior of the probe 1 will overheat and the shielding on the wire will melt once again or the filter R will be damaged, again requiring disassembly and reassembly.
Even still another problem with prior art Langmuir probes 1 is that the current flowing through the wiring W fluctuates, creating RF xe2x80x9cnoisexe2x80x9d. An RF induction filter R helps remove some of the noise, but much of the noise remains, which makes taking precise measurements difficult. Further, the filter R cannot be tuned to block different frequencies. Rather, the filter R must be removed and another filter that blocks the desired frequencies installed.
The present invention is a plasma probe that is much more heat tolerant than prior art Langmuir probes. Conduction and convection are utilized to remove heat from the interior of the probe, thereby reducing the occurrence of melted wiring and heat damage to filters. Also, the present invention assembles much more easily than known probes. Furthermore, the present invention uses capacitance to overcome the limitations of the prior art with respect to the filtering of noise when taking readings with the probe.
The present invention includes rigidly attaching the connector guide to a second connecting portion (e.g., a female pin guide) of a plasma probe to align the second connecting portion with a first connecting portion (e.g., a male pin guide). The guide has an outer diameter that is almost equal to but slightly smaller than the inner diameter of the shell. As the wiring is pushed into the shell, the guide slides along the interior of the shell and guides the second connecting portion into attachment with the first connecting portion. Thus, rapid assembly and disassembly are possible, permitting even routine maintenance to be performed more quickly than in the past.
As noted previously, the current flowing through the wiring fluctuates, making taking precise measurements difficult due to inductive effects. In an embodiment of the present invention, the guide is constructed of an electrically conductive material to provide a capacitance between the guide and plasma outside the probe. That is, the guide forms one plate of a capacitor, the plasma forms another plate, and the shell acts as a dielectric. In this way a large capacitance is created which filters the variations in current to reduce the noise developed on the signal traveling through the wiring of the probe.
To reduce the damage caused by heat in the interior of the shell, the guide may also act to cool the second connecting portion and the nearby wiring, thereby reducing the probability of meltdown of the components of the wiring and helping to prevent oxidation of the first and second connecting portions. In such case, the guide is preferably thermally conductive to guide heat away from the second connecting portion. To increase the cooling effect of the guide even more, at least one cooling fin may extend from a rear face of the guide.
To enhance the cooling function of the guide, or used without the guide, a coolant inlet line may be inserted in the shell. The coolant inlet line injects coolant (preferably air) into an interior of the shell to provide convective cooling for the guide and wiring in the interior of the shell. The coolant may comprise air or another substance such as nitrogen gas. Optionally, a coolant outlet line may be inserted in the interior of the shell to assist the escape of coolant from the interior of the shell. For maximum cooling effect, an opening of the coolant outlet line should be positioned towards an end of the shell opposite the first connecting portion so that the length of the interior of the shell and the internal components therein are cooled. The coolant inlet line may be coupled to the wiring before inserting the second connecting portion and the guide into the shell of the probe during assembly.
Before inserting the second connecting portion and the guide into the shell of the probe, the first connecting portion may be cleaned to remove any oxidation caused by overheating or simply from general use. A cleaning device that can be used has an elongate shaft with an open end and an abrasive inner lining positioned towards the open end of the shaft. The abrasive inner lining is rubbed on the first connecting portion for cleaning unwanted material from the first connecting portion.
An advantage of embodiments of the present invention is that the internal components of the probe can be more easily removed and replaced than was heretofore possible. This is particularly useful when performing periodic maintenance or replacing components.
Another advantage of embodiments of the present invention is that the capacitance created by the guide reduces signal noise. This allows measurements to be taken which are much more accurate than ever before.
Yet another advantage of embodiments of the present invention is that the internal components of the probe last much longer due to internal convective cooling of the components. Furthermore, signal noise is reduced due to a lower operating temperature of the probe.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing.